Motor vehicles typically operate using an internal combustible engine to convert the energy in a combustible fuel, such as gasoline or diesel, into mechanical energy to drive the wheels and otherwise operate the motor vehicle. Unfortunately, combustible fuels are expensive and contribute to environmental pollution. Due to these drawbacks, attention has been given to the problems of reduction of fuel consumption and pollutants emitted by automobiles and other highway vehicles.
To alleviate some of these drawbacks, hybrid vehicles of various configurations have been proposed. For example, in some series hybrid vehicles, the vehicle batteries are used to power the wheels via an electric motor, and the internal combustion engine powers a generator and can be operated in its most fuel-efficient output power range while still allowing the electric traction motor (powered by the batteries or the generator) to drive the vehicle. In another configuration, commonly called a parallel hybrid vehicle, the internal combustion engine and electric motor are matched through a complex gear train so that both can provide torque to drive the vehicle. In a parallel hybrid vehicle, the vehicle can be operated in several different modes including a mode where the engine is run at constant speed and excess power is converted by a motor/generator to electrical energy for storage in the batteries. Other hybrid vehicle arrangements and modes of operation are also known.